1. Field of the Invention
Preferred embodiments of the present invention generally relate to a switching regulator, and more particularly, to a switching regulator configured to be operated in a PWM control mode or a PFM or VFM control mode according to a load connected to an output terminal of the switching regulator.
2. Discussion of the Related Art
Recently, environmental concerns have resulted in increased demand for power savings in the use of electronic devices. This demand is particular acute for battery-powered electronic devices.
In general, in order to save power, it is important to reduce power consumed by an electronic device and eliminate wasted power consumption by improving efficiency of a power supply circuit.
As a high-efficiency power supply circuit used for a small electronic device, a non-isolated switching regulator employing an inductor is widely used, for which three control modes, a PWM (Pulse Width Modulation) control mode, a PFM (Pulse Frequency Modulation) control mode, and a VFM (Variable Frequency Modulation) control mode, are commonly known.
In the PWM control mode, an output voltage output from a switching regulator is controlled to be constant by varying a duty cycle of a clock pulse having a constant frequency.
In the PFM control mode, an output voltage output from a switching regulator is controlled to be constant by varying a frequency of a clock pulse having a constant pulse width.
In the VFM control mode, an output voltage output from a switching regulator is controlled to be constant by controlling an output of a clock pulse having a constant pulse width in accordance with an error in the output voltage.
More precisely, the PFM control mode employs two methods, a method to vary the frequency of the clock pulses steplessly and a method to vary the frequency of the clock pulse artificially by thinning out the clock pulses having the constant frequency used in the PWM control mode.
Since the PWM control mode performs an on-off control on a switching transistor at a constant frequency even when a load connected to a switching regulator is light, efficiency in driving a light load to which a low current is supplied with the PWM control mode decreases. On the other hand, in the PFM or VFM control mode, a frequency of a signal for switching the switching transistor varies in accordance with a load connected to a switching regulator. As a result, even though noise or voltage ripple affects the load substantially, the PFM or VFM control modes drive a light load more efficiently than the PWM control mode does.
Conventionally, by switching a control mode of the switching transistor between the PWM control mode and the PFM control mode or between the PWM control mode and the VFM control mode in controlling a switching regulator in accordance with a load condition connected thereto, efficiency in supplying power increases irrespective of light and heavy loads.
As for a method to detect a load condition required for such control, a method to detect an output current from an output terminal by inserting an output current detection resistor between a power supply as an input voltage and the output terminal is generally used. However, in such a method, since power loss at the output current detection resistor increases as the output current increases, the method is not appropriate for a small electronic device using a battery as a power supply.
In order to solve the above problem, for example, another method that does not employ an output current detection resistor has been proposed. In this method, a load condition is indirectly detected by using a voltage level of an error amplifier.
However, such a technique has a drawback in that an output current to determine the control mode cannot be measured correctly due to an integration circuit included in the error amplifier. Normally, in order to remove an effect of a ripple component superimposed on an output voltage, an integration circuit is added to an error amplifier as a phase compensator.
In general, the integration circuit is optimized to an operation frequency in the PWM control mode. On the other hand, in the PFM control mode, the integration circuit effectively functions right after the control mode is changed. However, when an operation frequency of the PFM control mode is set lower than in the PWM control mode or is lowered by thinning out several clock pulses from clock pulses for the PWM control mode and an on/off operation by a switching transistor is interrupted due to the lower frequency, since an output voltage from the integration circuit is also an output voltage from the error amplifier, the output voltage from the error amplifier becomes zero or close to the input voltage, and the integration circuit does not work effectively as a circuit for detecting an output current. As a result, in the PFM control mode, the error amplifier cannot keep a constant output voltage with respect to the output current, and a relation between the output voltage from the error amplifier and the output current cannot be kept constant either.
Consequently, a problem arises in that the output current to determine the control mode can be measured in the above method less accurately than in the method using the output current detection resistor.